1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive for polarizing plates, a pressure-sensitive sheet, a polarizing plate with the pressure-sensitive adhesive and a production process for the same, and an optical film using the above polarizing plate with the pressure-sensitive adhesive and a production process for the same. More specifically, the present invention relates to a pressure-sensitive adhesive for polarizing plates which is suitably applied to a polarizing plate, particularly a polarizing plate integrated with a viewing angle expanding film and the like or laminating a polarizing plate on retardation film and can adhere the above polarizing plate on a liquid crystal cell with good adhesion durability and which has the characteristic that a liquid crystal display obtained is less liable to cause light leakage even under the environment of high temperature and high humidity and can readily be peeled off when applied mistakenly, a polarizing plate with the above pressure-sensitive adhesive and a production process for the same, and an optical film and a production process for the same.
2. Description of the Related Art
When a sheet comprising organic materials is applied on an adherend such as glass, ceramic and metal via a pressure-sensitive adhesive, the unfavorable situation that peeling of the sheet in an end part or lifting thereof is caused with the passage of time has so far frequently been brought about.
In order to solve the above situation, usually used is a strongly pressure-sensitive adhesive material in which an adhesive performance is enhanced by raising a molecular weight of a component constituting the pressure-sensitive adhesive or elevating a cross-linking density thereof. However, while the holding power is improved when using the above strongly pressure-sensitive adhesive material, the pressure-sensitive adhesive can not follow a change in the shape which is caused by shrinkage or swelling of the sheet comprising organic materials under the environment of high temperature and high humidity, and it is a factor to bring about various troubles.
On the other hand, ones in which a polarizing plate is applied on a surface thereof are included in optical parts, and a representative example thereof includes a liquid crystal cell of a liquid crystal display (LCD). It shall be explained below with reference to FIG. 1.
This liquid crystal cell 13 has usually a structure in which two transparent electrode substrates having orientation layers formed thereon are disposed with the orientation layers turned to an inside so that a prescribed interval is formed by spacers, in which the periphery thereof is sealed to interpose a liquid crystal material in the above interval and in which polarizing plates 11 are disposed respectively on the two transparent electrode substrates described above via pressure-sensitive adhesives 12. In general, the polarizing plate described above comprises a polarizing film having a three layer structure in which optically isotropic films, for example, triacetyl cellulose (TAC) films are adhered on both surfaces of a polyvinyl alcohol base polarizer, and a pressure-sensitive adhesive layer is further provided on the other surface thereof for the purpose of applying on an optical part such as a liquid crystal cell.
Further, as shown in a schematic drawing of FIG. 2, a retardation film 24 is disposed in a certain case between a polarizing plate 21 and a liquid crystal cell 23 via pressure-sensitive adhesives 22 and 25 in order to improve the viewing angle characteristics.
When applying a polarizing plate having the constitution described above on an optical part such as a liquid crystal cell or applying a polarizing plate to a retardation film, it results in assuming a multilayer structure comprising different kinds of the materials and is short in a dimensional stability, and a change in the dimension caused by shrinkage and swelling is increased particularly under the environment of high temperature and high humidity. In general, the strongly pressure-sensitive adhesive described above is used as a pressure-sensitive adhesive for the above polarizing plate, and therefore lifting and peeling brought about by a change in a dimension of the polarizing plate can be inhibited. However, a stress brought about by a change in a dimension of the above polarizing plate can not be absorbed by the pressure-sensitive adhesive layer to make a residual stress in the polarizing plate uneven. As a result thereof, brought about are the problems that liable to be produced are so-called “light leakage” in a TN liquid crystal cell and “uneven color” in STN liquid crystal.
In order to solve the problems described above, disclosed is a technique in which a pressure-sensitive adhesive is suitably softened and provided with a stress relaxing property by adding a low molecular weight substance such as a plasticizer to the pressure-sensitive adhesive (refer to, for example, a patent document 1). However, addition of the low molecular weight substance causes stain of the liquid crystal cell when peeling off the polarizing plate and, in addition thereto, lowers the holding power, and lifting and peeling brought about by the passage of time are liable to be caused.
On the other hand, disclosed is a pressure-sensitive adhesive sheet obtained by radiation-induced cross-linking a mixture comprising an acrylic base copolymer and an acrylic base polymer having a radiation-polymerizable group in a side chain in a proportion of 100:1 to 100:100 in terms of a weight ratio (refer to, for example, a patent document 2).
In the above patent document, examples are described for polarizing plates, but a size of samples evaluated for a light leaking property is not described in any of the examples. The present inventors evaluated a light leaking property of a polarizing plate having a 15 inch size using the pressure-sensitive adhesives described in the examples of the above patent document, and the results thereof were not sufficiently satisfactory.
As described above, it is difficult to allow an adhesion durability to be consistent with a light leakage preventing property in a pressure-sensitive adhesive for a polarizing plate, and it is a subject to allow them to stand together.
In particular, when a severer durability such as in automobile uses is required, it is more difficult to allow an adhesion durability to stand together with a light leakage preventing property.
In a production step of a liquid crystal display and the like, when an applying position is deviated in applying a polarizing plate on an optical part such as a liquid crystal cell, the polarizing plate has to be peeled off in a certain case after some time passes since applied to reuse the expensive liquid crystal cell. Accordingly, required is such a pressure-sensitive adhesive that a polarizing plate can relatively readily be peeled off from a liquid crystal cell even after some time passes since applied on the liquid crystal cell via the pressure-sensitive adhesive coated on the polarizing plate. This is a property which is contradictory to strengthening of an adhesive strength for providing an adhesion durability, and it is a subject to allow them to stand together.    Patent document 1: Japanese Patent No. 3272921    Patent document 2: Japanese Patent Application Laid-Open No. 107005/2001